Cytokinins biological activity

Polymeric forms of ACC 71 were synthesized and their biological activities evaluated. Synergistic interactions between ACC polymers and the cytokinin 6-N-benzylaminopurine have been indicated [120]. 

Mixed Cytokinins. The first cytokinin, kinetin [525-79-1] (3), was isolated from stale herring sperm (8) but, like so many biologically active natural products, it was later found in the vascular system of tobacco stems and leaves (9). Yeast also proved to have a very high titre of kinetin (see Yeasts) (8). The compound is very active in increasing cell division in tobacco wound callus tissue that has been cultured on White s agar medium supplemented with 2 mg/L of indole-3-acetic acid (IAA) [87-51 -4], The presence of IAA is mandatory to induce cell division in the presence of kinetin. 

There are good reasons presently for believing also that free cytokinins have important biological activity independently of any direct association with tRNA (1) there are results showing direct effects of exogenous cytokinin independently of any apparent incorporation into tRNA (2) ethanolic extracts of corn kernels contain the trans-isomer of zeatin, while the tRNA hydrolysates of corn kernels contain the cis-isomer, suggesting that zeatin is not a precursor in the synthesis of tRNA in that material (3) dihydrozeatin... 

Cytokinin specificity is determined by the structure of its side chain, where even a small modification can significantly affects the biological activity. Among enzymatic modifications that have been already proved in vivo are ranked O-glycosylation and O-acetylation of zeatin, cisJtrans isomerization of its hydroxyl group, double bond reduction to dihydrozeatin and hydroxylation of A -isopentenyl moiety of isopentenyladenine type to zeatin-types of cytokinins. 

Cytokinins, like auxins, have a spectrum of biological activities (Table 2). They were first recognized because of their ability to cause isolated plant cells, when auxin was also present, to undergo cell division so as to produce a callus [16]. From this has developed the dogma that cytokinins are required for all mitoses in plants. In fact, there is only... 

Cytokinin degradation via side chain cleavage is another process regulating levels of biologically active cytokinins in plant cells. Unlike other metabolic steps the cleavage of the side chain from the cytokinin molecule results in an irreversible destruction of cytokinin structure, which is of course associated with a complete loss of biological activity. 

Biologically active auxins are the only agents known to induce expression of the SAUR genes in soybean. By contrast, the Arabidopsis SAUR-ACl transcripts are reported to accumulate in response to cytokinin as well as auxin [67], but it has not been shown that the cytokinin response is transcriptional or posttranscriptional. Cytokinins actually decrease SAUR transcripts in soybean when applied alone or in combination with 2,4-D... 

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